Cell death is a critical regulatory process in development, in homeostasis, and in disease. Many cell death triggers have been identified, although little is known about many of the molecular pathways that lead to cell death. Galectin-1, a member of a family of evolutionarily ancient lectins, induces death of thymocytes and T-cells. In mammals, galectin-1 is expressed in lymphoid tissues, at sites of inflammation, and in some types of cancer. Galectin-1 has also been shown to be immunosuppressive in several animal models of autoimmune disease. Galectin-1 induces cell death of transformed epithelial cells as well, so that galectin-1 mediated cell death may be a fundamental mechanism that regulates cell survival in many tissues. Other galectin family members have pro- or anti-apoptotic activities in different tissues, suggesting that different galectins may cooperate to regulate cell death. The goal of this application is to characterize the molecular mechanism of galectin-1 cell death, to understand the interaction of galectin-1 with glycoprotein receptors on the cell surface, and the downstream events that regulate cell death. The Specific Aims are: 1. To define features of the T-cell surface receptors for galectin-l required to transmit the death signal. CD7 and CD43 are glycoprotein receptors for galectin-1. Features of the glycans and the polypeptides that are essential for sending the death signal will be identified. 2. To examine how galectin-l binding to T-cells results in molecular interactions to deliver the death signal. Galectin-1 binding results in a unique pattern of receptor reorganization on the T-cell surface. We will identify features of the receptors required for this interaction, and investigate whether receptor reorganization is required for cell death. Effects of galectin-1 on cytoskeletal linker proteins and on the actin cytoskeleton will be characterized. To examine cell-cell interactions that govern T-cell death, receptor reorganization during galectin-1 mediated binding of T-cells to thymic stromal cells will be examined. 3. To characterize intracellular components that regulate the galectin-l cell death pathway. We will examine the roles of the protein kinase C and protein phosphatase families of enzymes in regulating galectin1 cell death. We determine how galectin-3 expression regulates T-cell susceptibility to galectin-1.